Solid state chemistry plays a major role in modern electronic-device technology, since the production of device components requires the preparation and manipulation of high-purity conductors, semiconductors, and insulators. New materials and techniques often lead to the development of new and improved devices, and the increasingly sophisticated requirements of the electronics industry provide a focus for research in solid state chemistry. A particular area that has provoked considerable interest recently is the conductor/semiconductor interface. Since electronic devices must communicate with one another and with the outside world via conducting contacts, the optimization of circuitry will require a high degree of chemical control over metallizations on semiconductors.
Presently, however, the typical Ohmic contact or Schottky barrier is produced by depositing some portion of the periodic table onto a device structure and annealing the result. Which portion of the periodic table is used as the contact metal and the details of the forming procedure are determined empirically by screening the metallizations for desirable electronic properties, such as Schottky barrier heights or contact resistance. Compared to the control and care exercised during the production of the semiconductor portion of the device, contact metallization is still in a primitive stage of development.
Conducting contacts to compound semiconductors are a particular problem. When an elemental metal is deposited onto a binary compound semiconductor substrate, a solid ternary system is formed. In general, the metal-semiconductor system will not be in thermodynamic equilibrium, and a chemical reaction will take place that involves the three elements of the system. The phase rule predicts that, for an arbitrary pressure and temperature, there may be as many as three different solid phases in equilibrium with one another in a ternary system. To appreciate why this fact is a cause for concern, one should consider the nature of the contact metallization after a reaction has occurred at the metal/semiconductor interface.
Early in the reaction sequence, the metal may form Q different compounds or alloys with each of the elements of the compound semiconductor. Even if the annealing process proceeds to a state of chemical equilibrium between the overlayer and the substrate, the chances are good that the resulting overlayer will be comprised of two different phases. Such a conducting film will be very heterogeneous, since the chemical and electrical properties of the two phases will most likely be very different, and controlling the morphology of the film will be extremely difficult. A schematic view of such a contact is shown in FIG. 1. In FIG. 1 the ideal contact is a uniform conducting phase. In practice, alloyed or sintered contacts on compound semiconductors are composed of at least two phases. The interface morphology is rough, and voids are often present. The film is extremely non-uniform and often breaks open. The semiconductor material under such a contact contains a large number of defects.
This situation contrasts with that for elemental semiconductors such as Si onto which a single-element containing metal has been deposited. Since a binary system will have at most two solid phases at equilibrium, the metal contact of an elemental semiconductor in thermodynamic equilibrium will be comprised of only one phase, either the unreacted metal or a compound such as a silicide. Thus, contact technology for compound semiconductors is in principle more troublesome than that for elemental semiconductors because of the extra degree of freedom introduced into the phase diagram of the metal/semiconductor system by the presence of an "extra" element.
There are other reasons to be concerned about the chemical reactions in metal/compound-semiconductor systems. Such solid state reactions consume some of the semiconductor material, and may seriously alter the morphology of the substrate material, as well as generate numerous defects in the metal/semiconductor interface region. These defects may later diffuse to active regions of the device, degrade its performance, and shorten its useful life. Another problem is that the system may not have achieved thermodynamic equilibrium during the forming process. If chemical reactions continue to occur after the device is placed in service, perhaps driven by the power dissipated in the device by the flow of current, the nature of the contact, and thus the device properties, will change with time. All of the above possibilities are highly undesirable effects, especially for very small devices and for systems that are intended to operate at high temperatures.
Studies have shown that films formed by thermal oxidation processes yield systems that are locally in thermodynamic equilibrium. For the case of GaAs, the film formed in contact with the semiconductor is composed of elemental As and Ga.sub.2 O.sub.3. After a sufficiently thick layer has grown, another oxide layer may form, containing other phases that are in equilibrium with the first layer. Thus, these oxide films may actually be multilayered structures, with the phases at each interface in local thermodynamic equilibrium. Behavior similar to that of the oxides may Q be expected from metal films, although the metal films are more complex since the metallic phases formed often have appreciable solubility with one another.
The stabilization of interfaces between metals and III-V compound semiconductors is especially desirable (e.g., whether metal=Pt, Ag, Au; III=Al, Ga, In; V=P, AS, Sb) because of the common use of these metals in integrated circuit fabrication.
Studies have described "out-diffusion" of the group III element through the metal overlayer or "interdiffusion" of the metal and compound-semiconductor materials during heat treatment, and have shown that this intermixing of the group-III metal and the overlayer metal is the result of a chemical reaction in which various alloys and intermetallic compounds are formed.
The general pattern that arises from these studies is that heating a metal/III-V system leads to the decomposition of some of the substrate material, with, in the case of Au, the simultaneous formation of Q Au/group-III intermetallic compounds and elemental group V, some of which may escape from the film into the vapor phase. The resulting conducting contact has several undesirable properties, the most obvious of which is a rough film morphology that consists of polycrystalline grains of intermetallic compounds, elemental group V, and the III-V compound-semiconductor material. Such a heterogeneous overlayer is not suitable for defining very small features for microcircuits. Another problem is that the Au-rich intermetallic compounds observed to form are brittle and have a higher resistivity than Au. Finally, defects introduced by such alloying procedures have been shown to affect adversely the lifetime of operating devices.
The occurrence of strong chemical interactions between Au and III-V compound semiconductors has also been observed near room temperature for thin films deposited on atomically clean surfaces In-vacuo photoemission and Auger studies of evaporated thin Au films (&lt;.ANG.100 .degree. A) on GaAs, GaSb, and InP substrates reveal that some group-III and sometimes group-V species appear on the surface of the evaporated film, even for film thicknesses of &gt;20 monolayers of Au. These chemical reactions at the Au/III-V semiconductor interface are probably the driving force in creating the defects that have been invoked as the cause of the Fermi-level pinning observed in these systems. Furthermore, these reactions generate elemental group-V species, which may be responsible in part for the "common anion rule" of Schottky-barrier formation on III-V compound semiconductors.
The formation of Au-Ga intermetallic compounds and As from Au films on GaAs may be understood in terms of simple thermodynamics. In a closed system, i.e., one in which no gas phase species are allowed to form, there is no detectable bulk reaction between Au and GaAs. This means that in the Au-Ga-As ternary phase diagram, Au and GaAs are connected by a tie-line, and are in thermodynamic equilibrium with one another. This situation is actually fairly rare. For Au and InP or Pt on GaAs, bulk chemical reactions will occur even in closed systems to form intermetallic compounds.
For open systems, i.e., for contacts that are annealed in vacuum chambers and/or operated in ambient atmospheres, there is an additional driving force for the reaction between Au and GaAs, the free energy involved in the evaporation of As.sub.4 or As.sub.2 from the film. For this case, the energetics of the reaction may be illustrated with a particular example: the reaction of Au with GaAs to form AuGa and gaseous As.sub.4 : EQU Au.sub.(s) +GaAs.sub.(s) .fwdarw.AuGa.sub.(s,1) +1/4As.sub.4 (g).multidot.(1)
FIG. 2 shows the change in Gibbs free energy (.DELTA.GR) of reaction (1) as a function of temperature, as well as the .DELTA. G.sub.R 's for EQU Au.sub.(s) +GaAs.sub.(s) .fwdarw.AuGa.sub.(s,1) +As.sub.(s,1), (2)
and EQU GaAs.sub.(s) .fwdarw.Ga.sub.(1) +1/4As.sub.4 (g).multidot. (3)
The Gibbs free energies plotted in this fashion were taken from the literature for the free energy of formation of GaAs and estimated using the enthalpy of formation for AuGa. Eq. (1) is plotted assuming that the vapor pressure of As.sub.4 is maintained at 10.sup.-8 Torr over the sample. The plot in FIG. 2 indicates that, at temperatures over .about.500 K, the overall reaction in Eq. (1) will proceed in the forward direction until either all the Au or GaAs has reacted. Although the temperature at which .DELTA.GR of Eq. (1) becomes negative is only an estimate, because of the uncertainties in the quantities used to derive the plot in FIG. 2, the primary point of interest is that depositing Au on a GaAs surface will lead to a breakdown of GaAs and evolution of As.sub.4 at a significantly lower temperature than that observed for the non-congruent sublimation of As.sub.4 from clean GaAs. Thus, when depositing Au on GaAs and annealing the resultant system in a large-volume container to temperatures .about.500 C, one would expect the Au to react with GaAs to form one or more Au-Ga intermetallic compounds with the subsequent release of As.sub.4 (g) On the phase diagram, the system would become increasingly As-deficient, and the equilibrium state would depend upon the total amounts of Au, Ga, and As present after the reaction reached completion This result is obviously undesirable.
Accordingly, it is the principal object of the present invention to thermodynamically stabilize a conductor/compound semiconductor interface.
It is a further object of this invention to provide a stable interface between an intermetallic compound and a compound semiconductor.